This invention relates to transmitter and receiver units for garage door actuators and more particularly to novel elements for tuning the frequency of the oscillatory circuits of these units.
Garage door actuators of the prior art have generally used variable capacitors in the oscillatory circuits thereof, i.e., the tank circuit in the transmitter unit and the tuning circuit in the receiver unit, in order to tune each of these units to operate at the same resonant frequency. Such variable capacitors are relatively expensive components for use in garage door actuators.
A variable inductor comprised of a coil provided with a movable ferrite slug for tuning is sometimes used in the tuning circuits of the receiving units of conventional radio systems. However, when using the specified band of ultrahigh frequencies allocated for garage door actuators by the Federal Communications Commission, such ferrite slug tuned coils are not practical for tuning because the characteristics of the tuning circuit are affected by the capacitance that is introduced by the ceramic-like materials used to form the ferrite slugs.